Asynchronous data transmission over audio communication systems typically involves sending frames of data segments, with the data being encoded into a modulated audio frequency carrier signal and prefixed with a synchronization signal. A correlator is then often used at the receiving end to determine the start of the data segment; that is, the start of the modulated carrier signal, so that the receiving modem can know where in the signal to begin decoding data. The received frame of data is cross-correlated to a known (reference) synchronization signal and peaks indicating high cross-correlation are then used to indicate the location of the beginning of the data payload.
In wireless telecommunications system, such as a CDMA cellular network, a vocoder is used to compress the inputted speech to achieve efficient wireless transmission of audio over a voice channel of the system. For digital (non-speech) data sent via the voice channel, particular encoding techniques are used to encode the data into a modulated carrier signal that can be successfully sent via the vocoder without loss of the digital data. Suitable techniques may be dependent upon the particular vocoder used. The success of transmission can depend, in part, on the ability of the receiving modem to determine the correct synchronization sequence location. For modulated digital data sent via the vocoder, the transmission can include noise introduced by the vocoder during the encoding process, raising the possibility that an incorrect position in the received data frame might produce the highest cross-correlation with the reference synchronization signal, and thereby resulting in an erroneous determination of the start of data. Furthermore, the data segment from one frame to the next might have different attributes, such as length and, although this length information could be added to the header or otherwise included in the data segment, doing so increases the overhead of the data segment.
Thus, there is a need for a data transmission approach for use with an audio communication system that provides reliable identification of the start of the data payload as well as an identification of one or more attributes of that payload.